This invention relates to the production of Lithium Iron phosphate (LiFePO4)-based powders as active cathode materials in Li-ion and Li rechargeable batteries.
There is an immediate need for reliable, safe, and non-toxic rechargeable batteries with high energy density, high power density, good shelf life, and low cost, for use in electric vehicle type applications. Such batteries can also be used in other commercial applications such as, wireless communication devices, camcorders and laptop computers. Rechargeable Li-based batteries, particularly rechargeable Li-ion batteries, are becoming the system of choice because of their overall good performance and high energy density. Presently, a majority of commercial Li-ion batteries use coarse LiCoO2 as cathode material; however, LiCoO2 has poor thermal stability and is toxic, rendering them unsuitable for large-sized battery applications, such as electric and hybrid vehicles, that require batteries to be stable, economical and environmentally friendly, along with good performance.
LiFePO4 has an ordered olivine type structure (olivine phase) and has recently been investigated as an attractive cathode material because of its high theoretical capacity, 167-171 mAh/g, low cost and non-toxicity. FIG. 1 shows the olivine structure, where chains (along the c direction) of edge-sharing transition metal—octahedra are connected to one another by phosphate tetrahedra. These (FePO4)− tetrahedral are connected to one another by octahedrally coordinated lithium atoms along the b axis [A. K. Padhi, K. S, Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc., 144, 1188 (1997)]. Among all olivine LiMPO4 compounds (M=Co, Mn, Fe, Ni and V), LiFePO4 has been studied most extensively, since the demonstration by Padhi et al. that it is possible to fabricate electrochemically active LiFePO4 compounds. Later on, Yamada et al. [A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., 148 (3), A224 (2001)] prepared coarse LiFePO4, and showed that it is possible to achieve a capacity of ˜160 mAh/g at a low current density. This data suggests that LiFePO4 cathode material has the potential to be a good candidate for Li-ion batteries.
However, the electronic conductivity of LiFePO4 is poor, and it is in the range of 10−7-10−8 S/cm. In order to utilize this material in commercial Li-ion batteries, the electronic conductivity of this material needs to be improved. The electronic conductivity problem is ameliorated either by coating the LiFePO4 particle surface with a thin carbon layer, by intimately mixing cathode particles with small-sized carbon particles, or by doping metals supervalent to Li+ [S.-Y. Chung, J. T. Blocking and Y.-M. Chiang, Nature Materials, 1, 123 (2002)] In the last few years, a large amount of research [H. Huang, S.-C., Yin and L. F. Nazar, Electrochemical and Solid-State Letters, 4 (10), A170 (2001); U.S. Patent Publication No. US2002/0192137A1; J. Barker, M. Y. Saidi and J. L. Swoyer, Electrochem. Solid State Lett., 6, A53, (2003); S. Frager, C. Bourbon and F. Le. Cras, J. Electrochem. 151, A1024 (2004); and Z. Chen and J. R. Dahn, J. Electrochem. Soc., 149, A1184 (2002)] has been done to produce LiFePO4/C composite materials using different methodologies.
Huang et al. [H. Huang, S.-C., Yin and L. F. Nazar, Electrochemical and Solid-State Letters, 4 (10), A170 (2001)] synthesized nanocomposites of LiFePO4 and conductive carbon by two different methods, which led to enhanced electrochemical accessibility of the Fe redox centers in this insulating material. In method A, a composite of phosphate with a carbon xerogel was formed from a resorcinol-formaldehyde precursor; in method B, surface oxidized carbon particles were used as nucleating agents for phosphate growth. They observed that electrochemical properties of powders prepared by method A were better because of the intimate contact of carbon with LiFePO4 particles. The resultant LiFePO4/C composite achieved 90% theoretical capacity at C/5, with good cyclability. In general, xerogels and aerogels have poor packing density, which lead to rechargeable Li-ion batteries with low volumetric densities. Additionally, the amount of carbon in the composite was about 15%, which is too high for use in commercial Li-ion batteries.
Chaloner-Gill et al. [U.S. Patent Publication No. US2002/0192137A1] described the production of nanoscale and submicron particles of LiFePO4 and LiFe1−xMnxPO4 (0.4≦x≦0) by a laser pyrolysis method. However, laser pyrolysis methods are relatively expensive processes, and powders produced by such processes are not suitable for cost conscious applications, such as electric and hybrid vehicles. S. Frager et al. synthesized LiFePO4/C composites using a mechanochemical activation method, which utilizes a high energy mill. It is generally believed that it is difficult to produce large quantities of powders at a low cost using a high energy mill process.
Barker et al. [J. Barker, M. Y. Saidi and J. L. Swoyer, Electrochem. Solid State Lett., 6, A53, (2003)] developed a carbothermal process. In this process, the transition metal reduction and lithium incorporation processes are each facilitated by a high temperature process, which is based on C═>CO transition. A mixture of LiH2PO4, Fe2O3 and C powders was pressed and heat treated at 750° C. for 8 hrs to form LiFePO4 powder. Although the carbothermal method is a single step process, the size of particles produced by this method is relatively large. SEM and light scattering data indicated significant amount of agglomeration in the powders. The average size of the agglomerates was ˜100 μm, and the size of the primary particle size was in the several microns range comparable to that of Fe2O3 precursor. Additionally, it has been observed that optimum performance from LiFePO4 powders can be obtained by reducing the particle size. Yamada and coworkers [A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., 148 (3), A224 (2001)] reported that LiFePO4 particles experience an abrupt increase in particle size and lose their capacity, if the reaction temperature is >600° C.